The present invention concerns a method of manufacturing diesel engine valves for both intake and exhaust having good corrosion resistance and strength.
In general, intake and exhaust valves for diesel engines are made of strong precipitation-hardening Ni-base heat resistant alloys represented by Nimonic 80A. It is a permanent problem to elongate valve lives, and there has been demand for further improvement in corrosion resistance and strength thereof. A typical process for manufacturing the valves conventionally practiced comprises hot forging at a temperature above 900.degree. C. to form valve blanks, and solution treatment followed by age-hardening.
It is, however, inevitable that the steps for improving corrosion resistance and strength of the valve materials results in lowering processability and increase of manufacturing costs, and therefore, the improvement has been effected to only the face parts which require better properties. For example, the assignee developed and disclosed (Japanese Patent Publication No. 64-8099) a valve for marine diesel engines made by using a strong precipitation-hardening heat resistant alloy as the material, forming valve cone parts by forging at a temperature in the range of 700-900.degree. C. under a forging degree of 20% or higher, and subjecting the forged products to age-hardening. It is also known to manufacture valves by forging at a temperature of 700-900.degree. C., and solution treatment followed by partial cold processing.
In the practice of the above technology proposed by the assignee, because of a relatively low forging temperature of 700-900.degree. C., cracking of materials at processing often occurs when the material is of low hot processability. Therefore, it is difficult to carry out forming with a high forging degree and to realize partial hardening to a desired high extent. In the other technology, in which partial cold processing follows solution treatment and age-hardening, the cold processing is the only way to increase strength, and unless strong processing is done at this stage, sufficient strength can not be obtained. However, limitation is posed on the forging degree at the cold processing to the valve blanks, which are already hardened to some extent by age-hardening, and thus, limitation is posed also on increase in the strength.
For the purpose of elongating valve lives not only strength but also corrosion resistance is an important factor. However, it is difficult to unite the strength and the corrosion resistance in the engine valves, because materials of high corrosion resistance generally have lower strength. Thus, it is concluded that, if partial strengthening of materials having good corrosion resistance could be made, this problem would be automatically solved.